I upgraded to a 15 Plus from an 11. I was very happy with the 11 and so far I'm very happy with the 15 Plus.
I can't comment on the battery because I haven't used it that much yet. It feels very responsive. I can't think of any quirks or issues so far, except that I had to call Verizon to activate it, probably because the activation servers were overloaded at the time.
I chose a black phone in a blue FineWoven case with the Belkin screen protector. So far so good.